DE102009056092B4 - Light source with a diode laser - Google Patents

Abstract

Light source with
a) an electrically freely triggerable diode laser (1) for generating light pulses;
b) a multi-stage optical amplifier (2, 3) for the light pulses generated by the diode laser;
c) a non-linear fiber (4) for the spectral broadening of the amplified light pulses;
d) a wavelength selector (5) for selecting parts of the spectrum of the amplified and spectrally broadened light pulses;
e) a controller (6) for controlling and monitoring the pump powers of all the optical amplifier stages (2, 3) such that a constant pulse repetition frequency of the light pulses ensures a constant pulse energy of the light pulses at the selected wavelength.

Description

Field of the invention

To measure the fluorescence lifetime of individual molecules, an excitation light source is required that generates pulses whose wavelength lies in the absorption spectrum of the substance under investigation and whose pulse duration is significantly shorter than the expected fluorescence lifetime. For fluorescence excitation of individual molecules in z. B. biological samples satisfy pulse energies of 10 to 20 pJ (pJ = picojoule).

For the specific investigation of molecular processes in living cells self-fluorescent proteins gain more and more importance. The absorption range of these modified proteins ranges from the blue to the orange / red wavelength range.

Blue, red and infrared excitation sources exist in the form of gain-switched diode lasers with pulse durations in the picosecond range. Its advantages are simple operation, freedom from maintenance and a freely selectable repetition rate with the possibility of triggering individual laser pulses as desired.

Although large parts of the spectrum from the near UV to the near infrared are covered by direct or frequency-doubled diode lasers, comparable sources of excitation between 530 nm and 630 nm are still lacking. It is precisely this field that is gaining in biological research with self-fluorescent proteins (Fluorescent Proteins) Green fluorescent protein (GFP), yellow fluorescent protein (YFP), mStrawberry, mCherry, mRaspberry [http://en.wikipedia.org/wiki/Fluorophore]). In addition, an excitation source with continuously tunable wavelength would be an important universal tool for basic research.

In addition to the selectable wavelength, an ideal excitation source for time-resolved fluorescence measurements would also have a freely selectable pulse repetition frequency.

On the one hand, to record fluorescence lifetime images, one would like to achieve the highest possible repetition frequency by the shortest possible measuring time. On the other hand, the period between two pulses must be long enough to be able to follow the fluorescence decay sufficiently. Since the fluorescence lifetimes vary depending on the dye studied, an adjustment of the repetition frequency is desirable depending on the measurement task.

If interactions between several dyes are to be investigated by means of Fluorescence Resonant Energy Transfer (FRET) (without errors due to incomplete FRET pairs), at least two excitation sources of different wavelengths must be synchronized with each other in a specific time regime (PIE-FRET = Pulsed Interleaved Excitation FRET). For this purpose, electrically freely triggerable lasers are particularly advantageous.

A supercontinuum, that is, white light, arises from the targeted exploitation of non-linear optical effects, such as Raman scattering, four-wave mixing, and self-phase modulation, which cause the spectrum to broaden. If short infrared pulses with a very high peak power are coupled into a special Photonic Crystal Fiber (PCF), then the interaction of the above-mentioned o. G. Effects broaden the spectrum to the entire visible range.

State of the art

Tunable pulsed lasers are produced in the visible spectral range using white light sources combined with acousto-optic tunable filters (AOTF) to isolate pulses. These systems are based on mode-locked fiber lasers, with whose emitted pulses a supercontinuum (white light) is generated. Due to the principle, such lasers work statically at a fixed repetition rate. Although the repetition frequency can be reduced by means of the acousto-optic modulators (so-called pulse pickers), an external triggering and thus synchronization of different laser sources for Pulsed Interleaved Excitation applications can not be realized thereby. Pulse pickers also usually have only a suppression of the pulses with a contrast of 1: 200, leaving a substantial background, which is extremely disturbing in fluorescence measurements.

Such light sources are z. B. described in the publications US 5,999,548 A . US Pat. No. 7,277,169 B2 . US Pat. No. 7,224,518 B2 . US 2006/0159398 A1 . DE 101 15 590 A1 and US 6 097 870 A , All these light sources are based on mode-locked femtosecond lasers with a fixed repetition rate.

The US 2008/0037108 A1 describes a pulsed diode laser operating in the so-called "gain switching" mode and generating broadband light using a nonlinear fiber. In the described laser, the repetition frequency is selected prior to start-up and can not be changed during operation. The pumping power to operate the laser is adjusted to the repetition frequency.

The US 2009/0097512 A1 describes an optical pulse source in which the light pulses are spectrally broadened by means of a nonlinear fiber. Subsequently, with the aid of a wavelength selector, arbitrary wavelengths can be selected from the broadband spectrum. In this way, narrowband pulses can be generated in a wide spectral range.

task

The present invention has for its object to generate laser pulses of a few hundred picoseconds pulse duration at a selectable wavelength in a predetermined time course.

solution

This object is achieved by the invention with the features of the independent claim.

• a) einem elektrisch frei triggerbaren Diodenlaser zur Erzeugung von Lichtpulsen;
• b) einem mehrstufigen optischen Verstärker für die von dem Diodenlaser erzeugten Lichtpulse;
• c) einer nichtlinearen Faser zur spektralen Verbreiterung der verstärkten Lichtpulse;
• d) einem Wellenlängenselektor zur Selektion von Teilen des Spektrums der verstärkten und spektral verbreiterten Lichtpulse; und
• e) einer Steuerung zur Regelung und Überwachung der Pumpleistungen aller optischen Verstärkerstufen derart, dass bei wechselnder mittlerer Wiederholungsfrequenz der Lichtpulse eine konstante Pulsenergie der Lichtpulse bei der selektierten Wellenlänge sichergestellt ist.

To solve the problem, a light source is proposed with:

• a) an electrically freely triggerable diode laser for generating light pulses;
• b) a multi-stage optical amplifier for the light pulses generated by the diode laser;
• c) a nonlinear fiber for spectral broadening of the amplified light pulses;
• d) a wavelength selector for selecting parts of the spectrum of the amplified and spectrally broadened light pulses; and
• e) a controller for controlling and monitoring the pumping power of all optical amplifier stages such that a constant pulse repetition frequency of the light pulses ensures a constant pulse energy of the light pulses at the selected wavelength.

A freely triggerable diode laser generates with each electrical signal a single light pulse at a certain wavelength with a pulse duration of picoseconds (ps) and a pulse energy of picojoule. The pulse energy and pulse shape are largely independent of the pulse period. Typical pulse durations are in the range of 10 to 300 ps, preferably 50 to 300 ps, more preferably 100 to 200 ps.

The light source is freely triggerable. Freely triggerable means that a single laser pulse can be triggered by the edge of an electrical control signal. The time sequence of the electrical control signals determines the time sequence of the laser pulses. Individual pulses can be generated, as well as any pulse sequences and periodically repeating pulse patterns. If the control signal is periodic in the ms range, after a few periods a highly stable output signal with respect to pulse energy and pulse width is established. In the currently customary freely triggerable diode lasers, minimum pulse intervals of up to 10 ns are possible.

In a subsequent multistage optical amplifier, the pulses are amplified several orders of magnitude to pulse energies of microjoules (μJ).

These very intense pulses of one wavelength subsequently broaden in a special nonlinear fiber (Photonic Crystal Fiber, PCF) to a spectrum of several hundred nanometers. The effect of spectral broadening depends strongly on the pulse peak power fed into the fiber. The use of pulses with approximately 100 ps results in a much more homogeneous spectral distribution compared to systems based on femtoseconds.

As a result, blue-shifted wavelength components in the range of 400 to 1,000 nm, preferably 450 to 800 nm, more preferably 500 to 700 nm, can be achieved.

The light source is not only freely triggerable, but also tunable. For this purpose, it has a wavelength selector for selecting parts of the spectrum of the amplified and spectrally broadened light pulses. As a wavelength selector z. As an AOTF (see above), prisms, grids or filters.

Furthermore, the light source is controlled by control electronics.

With the wavelength selector one or more narrow-band ranges can be filtered out of the generated supercontinuum, thus creating a freely triggerable, tunable light source.

The time interval between the pulses is infinitely variable and can be adjusted over a wide range.

By a plurality of individual triggerable lasers, which are each tuned to different wavelengths, sequences of pulses of different wavelengths can also be generated.

In this way, interactions between fluorescent dyes can be investigated, which differ both in their spectral properties and in their fluorescence lifetime.

The light source is suitable for use in the observation of fluorescence dynamics, ie for spectroscopy and microscopy, in particular for confocal microscopy, in particular for the analysis of fluorescence lifetime imaging (FLIM, fluorescence lifetime imaging), or for fluorescence correlation spectroscopy (FCS, fluorescence correlation spectroscopy). ,

The average power of the pulsed input laser depends linearly on the pulse repetition frequency with constant pulse energy. In the case of a freely triggerable input laser, this can cover a very wide range and change suddenly when the control signal changes. These changes must be countered with adapted pump powers in the optical amplifier.

The effect of the optical amplifier depends very much on the inversion, ie how many ions are in the excited state. With higher average input power, more pump light must be supplied, with low average input power, the pump power must be reduced to protect the amplifier from destruction.

Since the quality of the spectrum at the end of the nonlinear fiber strongly depends on the pulse peak power of the laser pulse, it must be ensured that this is as constant as possible and independent of the pulse repetition frequency. Therefore, not only to protect the amplifier but also to preserve the quality of the spectrum, the pump power must be adjusted to the average input power.

For this purpose, the size of the average power is monitored in several places. The pump powers of the individual amplifier stages are controlled by a microcontroller so that the overall structure is in an optimal operating state. The useful signal at the output of the wavelength selector can also be permanently monitored.

Further details and features will become apparent from the following description of preferred embodiments. In this case, the respective features can be implemented on their own or in combination with one another. The possibilities to solve the problem are not limited to the embodiments. For example, area information always includes all - not mentioned - intermediate values and all imaginable subintervals.

The embodiments are shown schematically in the figures. In detail shows:

1 a schematic representation of the light source;

2 a schematic representation of a light pulse train; and

3 a schematic representation of a light pulse sequence with different wavelengths.

construction

1 shows a diode laser 1 , The diode laser 1 uses a gallium arsenide multi-quantum well laser chip and emits infrared light in the range between 1000 and 1070 nm. The pulse lengths are in the range of 50 to 120 ps, with about 1 W peak power and pulse energies of up to 100 pJ.

The light of the diode laser 1 will be in a first stage 2 an optical amplifier coupled. The optical amplifier consists of a doped fiber amplifier, which are numerous described in the literature. Optical waveguides doped with ions of rare earth metals become intense by pumping light 21 put into an excited state. Stimulated emission caused by the pulsed input light causes the optical amplification effect.

Preferably, a two-stage ytterbium-dodded fiber amplifier (YDFA) is used 2 . 3 for use. YDFA have their gain maximum at 1030 nm and absorption maxima at 976 nm and 915 nm. The first stage 2 consists of a heavily ytterbium-doped single-mode fiber, in whose core 976 nm pump light is coupled by means of a Wavelength Division Multiplexer (WDM). The performance level 3 consists either of a double-core fiber, in which the pumping light is coupled into a second layer between the core and the cladding, or of a large-mode-area fiber (LMA-fiber) with a core diameter of 30 μm or more, but through a special hole structure in the cladding Possesses properties of single-mode fibers. The second amplifier stage 3 is powered by several pump diodes 21 at 976 nm or 915 nm and 10 to 50 W pump power.

As a result of this amplification, there are infrared pulses with peak powers greater than 1 kW.

The amplified light pulses are used for spectral broadening into a photonic crystal fiber (PCF) 4 coupled. In a photonic crystal fiber optical parameters can be changed by targeted arrangement of air holes in the fiber cladding. By means of a special chromatic dispersion in the fiber, various nonlinear effects (essentially the above-mentioned self-phase modulation, four-wave mixing and Raman scattering) can be exploited specifically for broadening the spectrum. Depending on peak power of the laser pulses and Length of the fiber, the spectrum widens more and more, starting from the injected wavelength of about 1060 nm, until the entire visible and near-infrared region is covered. Since the pulse energy influences not only the intensity but also the shape of the spectrum, the influence of the pulse energy of the infrared input pulse on the pulse energy at a certain visible wavelength at the end of the PCF is particularly large [sa WJ Wadsworth et al., Supercontinuum generation and four-wave mixing with Q-switched pulses in endlessly single-mode photonic crystal fibers ", Opt. Express 12 (2), 299 (2004)].

Typical fiber lengths are in the range of z. 0.1 to 10 meters or more; the length depends on the fiber type and the coupled pulse energy.

Finally, the white light pulse passes through the wavelength selector 5 ,

As a result, selectable wavelength pulses are achieved in the visible range, with pulse lengths in the range of 100 ps and pulse energies in the range of 1-10 pJ / nm.

control

Since the pulse energy of the output pulses depends strongly on various parameters of the preceding stages of the light source, various measures are taken to stabilize the pulse energy of the output pulses.

On the one hand, the average power of light at the output of each stage by means of suitable measuring devices 22 supervised.

On the other hand, the light source has a controller 6 on. The control 6 regulates and monitors the pumping power of all amplifier stages. This ensures that constant output conditions (pulse energy at the selected wavelength) are ensured under changing input conditions (in particular changing average repetition frequency). At various points, the average power at the output of the respective stage is measured by means of sensors and compared with reference values. The pump powers of the individual amplifier stages are readjusted accordingly. Experience-based control algorithms or possibly value tables for different operating modes ensure optimal operation at all times. The optional monitoring of the actual useful signal behind the wavelength selector additionally allows the compensation of inhomogeneities of the white light spectrum under changing conditions.

Examples of generated pulse patterns

2 shows one with the light source according to 1 can be generated at one wavelength. A sequence of pulse packets (so-called bursts) was produced 24 , A burst 24 consists of (in this example) four pulses 26 , Plotted is the light output P over time t.

Typical pulse intervals (also for periodic pulses) are in the range of 10 to 1,000 ns. Distances between the bursts are in the range of 100 to 10,000 ns.

Such a kind of burst sequences is important to B. Excite molecules by a short-term intense excitation (burst) in a particular state (eg triplet). The longer excitation pause following the burst allows the targeted restriction of the residence time in this state in order, for example, to minimize the excitation into higher triplet states [see, for example, US Pat. B. Donnert G., Eggeling C., Hell SW .: "Triplex relaxation microscopy with bunched pulsed excitation", Photochemical & Photobiological Sciences, Vol. 8 (4), pp. 481-485, 2009].

3 shows another with the light source according to 1 can be generated at two different wavelengths (solid and dashed lines). Again a sequence of pulse packets (so-called bursts) was produced 30 , A burst 30 consists in this example from each first two pulses 32 the first wavelength followed by four pulses 34 the second wavelength. Plotted is again the light output P over the time t.

This z. For example, the pulses of the first wavelength originate from a freely triggerable, pulsed diode laser, while the pulses of the second wavelength originate from a slave operated light source as described herein.

Between the individual pulses 32 . 34 exists z. B. a time interval of 12 ns. Between the bursts 30 there is a distance of z. B. 5 × 12 ns = 60 ns.

Such pulse patterns are necessary, for example, in the two- or multi-color excitation of FRET pairs in order to characterize the energy transfer between the FRET fluorophores and thus their distance.

A different number of pulses per wavelength can be used in order to realize different excitation powers even at maximum excitation power per pulse, yet integrated via the pulse sequence for different excitation colors. In this way, differences in the absorption coefficient or the detection efficiency can be compensated. It is also possible in this way to set special statistically relevant detection signal ratios [see, for example, US Pat. B. Zarrabi N., Ernst S., Düser MG, Golovina-Leiker A., Becker W., Erdmann R., Dunn SD, Börsch M .: "Simultaneous monitoring of the two coupled motors of a single FoF1-ATP synthase by three-color FRET using duty-cycle optimized triple-ALEX "; Proceedings of SPIE, Vol. 7185, pages 718505-718505-14 (2009)]. In addition, bursts of different colors for the reasons mentioned above for monochrome bursts should also be combined with stimulation pauses.

Similar pulse regimes are z. B. used in tissue spectroscopy, where different wavelengths quasi-simultaneously query different tissue properties.

LIST OF REFERENCE NUMBERS

1

diode laser

2

first stage of the optical amplifier

3

second stage of the optical amplifier

4

non-linear fiber for spectral broadening

5

wavelength

6

control

21

pump diodes

22

Medium power meter

24

burst

26

light pulse

30

burst

32

Pulses of the first wavelength

34

Pulses of the second wavelength

Claims (1)

Light source with a) an electrically freely triggerable diode laser ( 1 ) for generating light pulses; b) a multi-stage optical amplifier ( 2 . 3 ) for the light pulses generated by the diode laser; c) a nonlinear fiber ( 4 ) for the spectral broadening of the amplified light pulses; d) a wavelength selector ( 5 ) for selecting parts of the spectrum of the amplified and spectrally broadened light pulses; e) a controller ( 6 ) for controlling and monitoring the pump powers of all optical amplifier stages ( 2 . 3 ) such that at alternating average repetition frequency of the light pulses, a constant pulse energy of the light pulses is ensured at the selected wavelength.

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